Four veteran Space Shuttle Main Engines will benchmark the performance of the rookie Space Launch System (SLS) Core Stage in the final two critical tests of NASA’s Green Run campaign. After loading the stage with propellant for the first time and demonstrating in a countdown that it can satisfy engine start criteria, the final test will be a full-duration firing of the Core Stage that is tentatively scheduled for late-October.
Last fired nearly a decade ago, the four Aerojet Rocketdyne RS-25 flight engines in the stage are going through some of their final pre-firing checkouts in the new SLS environment. After another brush with tropical weather, Green Run testing resumed this week with the fifth of eight Green Run test cases, where Core Stage hydraulic systems will be used with previously tested avionics and propulsion systems to verify they are ready to support the stage’s first fueling and firing.
Veteran engines prepped to benchmark rookie stage debut
The four RS-25 engines installed in the first Core Stage flight article each flew several Space Shuttle launches while installed in the orbiter fleet. The SLS Core is a new four-engine rocket stage designed around the RS-25’s capabilities.
One of the main objectives of the Green Run campaign is an operational verification of the stage design, and the former Shuttle engines play a supporting role in the tests. The finale of the eight-test campaign is a hot-fire that is planned to run the stage for a little over eight minutes, fully emptying the tanks in a flight-duration firing demonstration.
Four of the eight test case sets are complete; along with the first-time stage operations in the test cases, these four Shuttle engines are going through their standard pre-firing preparations for the first time for SLS. The engines installed in the Core Stage in some cases haven’t been fired since Shuttle launches concluded in 2011; now that the new stage is nearing its first test-firing, the engines are going through their checkouts to verify they are ready to support the two big tests.
“From an engine standpoint, we’re doing nothing different than we did on Shuttle,” Bill Muddle, Lead RS-25 Field Integration Engineer for Aerojet Rocketdyne, said. “We do what we call an electrical checkout, there’s three categories there. There’s a sensor calibration and checkout, there’s an igniter checkout, and then there’s a DCU (digital computer unit) checkout.”
“Once we get that portion done, then we’ll bring up pneumatics to the engine and then we’ll run through a pneumatic checkout which is basically running through all the pneumatic controls on the engine. Then, once that’s complete, we’ll bring up hydraulics and then we’ll go do actuator calibrations and then we’ll get into the flight readiness test.”
Those procedures are part of the fifth Green Run test case that is currently in progress, where the systems that were activated during the previous test cases are now supporting checkout of the Core Stage hydraulics and the RS-25 engines. The stage hydraulics provide power for engine control and steering.
(Photo Caption: The B Test Stand on July 14, with Core Stage-1 installed in the B-2 position on the left. The stage’s four RS-25 engines will fire in the refurbished flame bucket below it on the left. The right-hand B-1 position of the stand supports single-engine RS-68 testing for Aerojet Rocketdyne.)
Valves in the engine are actuated hydraulically, with a pneumatic backup for shutdown; in the flight readiness test, each engine’s new controller will command the valves through their countdown and firing sequences. The new engine controller units (ECUs) are part of a new set of avionics being used on SLS.
Each engine has a dedicated, self-redundant ECU that controls its operation, monitors its health, and communicates with the launch vehicle flight computers. The new ECUs went through dozens of single-engine RS-25 firings on ground test engines as a part of a certification program completed in 2017.
Although these flight engines helped power several Shuttle launches, they will be using SLS operating conditions. The engine certification program verified the existing design was adaptable to the SLS requirements for operating temperatures, pressures, and power settings.
As with the old engine controllers, the new controllers have two, redundant digital computer units, each with their own control channel. Both channel A and channel B are exercised during the flight readiness test, along with the primary hydraulics and backup pneumatic capability for engine shutdown.
“It’s basically a simulation of hot-fire,” Muddle explained. “So we’ll go through the purge sequences of the engine, not the durations but just through the purge sequence, [and] verify everything.”
While the stage’s liquid hydrogen and liquid oxygen tanks are being filled with propellant, the engine hardware is chilled down to similar cryogenic temperatures; the engine is also purged to keep the sealing surfaces and other machinery free of contamination prior to ignition.
After going through a compressed countdown timeline, the flight readiness test then does a dry run-through of the firing, first on the channel B side of the controller.
“We’ll go run a start sequence, and then we’ll go into mainstage [where] we’ll run through several power levels to verify that the valves react to the request for the power change. And then we’ll do a pneumatic shutdown the first time,” Muddle noted. “And then the second time we’ll re-perform that same purge sequence operation, start, under channel A and then we’ll do a hydraulic shutdown.”
In another part of the on-going test case five procedures, the stage’s hydraulic thrust vector control (TVC) systems are being used to gimbal the engines. “[The] patterns are different than hot fire based on different requirements that need to be satisfied,” Muddle noted. Those procedures will help verify that the hydraulics are ready for more vigorous gimbaling activity during the hot-fire test.
In addition to testing and checkout of the TVC actuators that move the engines with their specialized avionics controllers, the vehicle hydraulic systems will also be configured for the upcoming hot-fire test. After the hydraulic reservoirs in each of the four systems are filled, ground hydraulics will be disconnected and vehicle hydraulics will be started just as they will be for the hot-fire and launch.
The Core Auxiliary Power Units (CAPU) are spin-started with gaseous helium fed through one of the ground umbilicals.
Test case five in progress after tropical weather delay
The first Core Stage flight article has been in the B-2 position of the B Test Stand at the Stennis Space Center in Mississippi since January. For a time in mid-August, weather forecasts predicted back-to-back close approaches from two Atlantic tropical weather systems within a few days of each other during the week of August 24, so NASA and Core Stage prime contractor Boeing postponed procedures for the fifth Green Run test case.
“We had started the facility preparation work for the Green Run Test 5, the hydraulics and thrust vector control testing, and we were planning to start the test on August 23,” NASA’s SLS Stages Office said in a statement. “Due to the prediction of two hurricanes in the Gulf Of Mexico with potential impacts to Stennis Space Center, NASA made the prudent decision to put the valuable Artemis I core stage flight hardware and B-2 test stand in a safe configuration.”
(Photo Caption: RS-25 engine 2063 on display at Stennis Space Center in February. The 16 “adaptation” engines use Shuttle-era hardware; the major component upgraded was the engine controller unit seen middle left here. The new engine hardware and software designed for SLS was certified to fly with the existing flight engine machinery in 2017.)
Hurricanes Marco and Laura ultimately did not directly impact the Mississippi space center, but a few days were lost while work at the test site stopped. Tropical Storm Cristobal also caused a work stoppage in June, but Boeing’s test team expected time would be lost to weather delays during the Green Run campaign and booked several days of margin into their overall schedule without knowing exactly when the interruptions would occur.
This has allowed the Core Stage schedule to remain more or less on track for a planned late-October hot-fire test since work on the stage resumed in May after a near two-month stand down for COVID-19 workforce safety.
Stennis fully reopened from Hurricanes Laura and Marco early on August 27, and vehicle and test stand systems were powered up on August 30 to pick up with testing.
Following test case five, the Green Run test team will start configuring the vehicle and stand for the final two, critical, fueled tests. Test case seven is the Wet Dress Rehearsal, which will be the first-ever propellant load of a Core Stage. Test case eight is the hot-fire test, repeating the countdown and propellant loading of the WDR followed by a planned flight-duration, eight-minute firing of the stage’s four engines.
While work starts on close-outs of the vehicle and stand for fueled testing, test case six is a countdown simulation planned to be performed with software updates for both the vehicle and ground control computers. The simulation will be a “dry”, unfueled dress rehearsal for the test team and program management before committing to fueling the stage for the last two tests.
Post-firing refurbishment at Stennis narrowed to expedite transport to KSC
After the hot-fire test is conducted, the stage will be prepared to go back on NASA’s Pegasus barge to be towed from Stennis through the Gulf of Mexico and then around the Florida peninsula to the Kennedy Space Center (KSC) for launch preparations. During the refurbishment period after the hot-fire test, the stage and its engines will be inspected and refurbished in the test stand. Part of the work is necessary prior to sea transportation, with the remainder to be performed at KSC in parallel with launch processing.
Between the hot-fire and launch, the engine hardware will be cleaned and inspected and parts replaced or repaired as necessary to prepare the engines for their next firing: launch. In order to shorten the amount of time between the hot-fire and shipment of the stage to KSC, some of the engine refurbishment work will be deferred until after arrival in Florida.
With the Artemis 1 launch behind schedule, NASA wants to deliver the Core Stage to KSC as soon as possible. Performing refurbishment and launch processing work in parallel at the launch site should be a more efficient use of time and allow the vehicle and programs to reach launch readiness sooner.
(Photo Caption: Core Stage-1 boattail, covered RS-25 engine nozzles, and Core auxiliary power unit exhaust ports seen in mid-May when essential personnel began returning to work after a two-month COVID-19 stand down.)
Additionally, most of the hands-on work on the SLS vehicle at KSC will be done indoors inside the large Vehicle Assembly Building at Launch Complex 39. In contrast, the stage is outside at Stennis, where weather conditions can be a more frequent disruption to work.
RS-25 prime contractor Aerojet Rocketdyne shortened their engine turnaround schedule from Shuttle to SLS based on the long flight and test history of the program. “The number of inspections between Shuttle and SLS are different,” Muddle said. “There’s actually less inspections now.”
“We’ve done great studies to try and help to minimize the refurbishment time because originally we started out at 90 days. We worked our way down to 42 now.”
“We’ve gone through the project multiple times, looking at all the inspections and everything else that needs to be done to turn an engine around,” he explained. “We started out with the Shuttle model, and so now we’ve gone through those iterations and gotten ourselves down to a lesser number of inspections and leak checks to where we’re at today which got us down to the 42-day refurbishment timeline.”
In addition to shortening the overall refurbishment timeline, Aerojet Rocketdyne also identified the post-firing work that needed to be done at Stennis versus work that could be “traveled” with the stage to KSC. “Based on the decision to go to KSC with the four critical locations, we knocked 19 days off the schedule,” Muddle said.
Back at Stennis, after the hot-fire test is complete, work platform decks will be swung down to provide close-up, hands-on access to the stage, including its dry volumes (forward skirt, intertank, and engine section) that are purged with nitrogen gas while the vehicle is fueled and then with breathable air when unfueled.
“Once the [propellant tanks] boil off and get inerted and it’s clear for people to come back onto the test stand, refurbishment starts,” Muddle added. “They start bringing in the decks, they start opening up the dry volumes, they’ll convert the dry volumes back over to air.”
As noted, four areas of post-firing work on the engines need to be completed in the stand at Stennis. “Drying is one of the four critical things that we have to do before we say that the engines are acceptable [to] pull the Core Stage out of the test stand,” Muddle explained.
(Photo Caption: The business end of Core Stage-1 in late January during B-2 installation activities. The two engines closest in this image (second and fourth from left) served on the last Space Shuttle launch in 2011 and will help power the first SLS flight.)
“One of the first things that you have to go do within 48 hours is to start drying the engines, getting moisture out of the critical areas of the engine.”
Second is to inspect and document the immediate condition of the engine hardware after firing. “We want to see if there’s any hot-fire related damage to the engine that we didn’t see on sensors or something like that,” he said. “We want to look at the exterior because once you start processing you get into the potential for collateral damage when they’re bringing platforms in and doing work around the engine. So we want to see the condition of the engine before we start working around that engine.”
Third is leak checks of all 1,080 coolant tubes in each engine nozzle; the tubes circulate liquid hydrogen to cool the inside of the nozzle from the hot, exiting, combustion gas.
Just in case any leaks are found, performing the nozzle tube leak checks in the stand at Stennis will give the program more time to decide what repair options they have, which ones to choose, and get set up at KSC to make the fixes after arrival. “There’s areas on that nozzle that could take a lot of time to get to [and] actually repair the leakage,” Muddle explained.
“So you want to know ahead of time before you send the vehicle to KSC whether the leakage that you saw and the areas that you saw can be repaired or can be accepted ‘use as-is.’ And if either one of those can’t happen, then the program has to decide whether the engine has to be removed.”
The fourth area is an internal inspection of the bottom of the engine nozzle. The aft manifold distributes the liquid hydrogen from the engine powerhead through the nozzle coolant tubes. “We have to go in and borescope around the bottom of the nozzle to look for some potential contamination, so that would be the fourth thing that we would want to do and knock it out before we head to KSC,” Muddle said.
Muddle noted that Aerojet Rocketdyne is planning for its RS-25 team to work around the clock to finish the post hot-fire task list at Stennis.
Lead image credit: NASA/Jude Guidry.